Coating formulations are commonly applied to a substrate by passing the coating formulation under pressure through an orifice into air in order to form a liquid spray, which impacts the substrate and forms a liquid coating. In the coatings industry, three types of orifice sprays are commonly used; namely, air spray, airless spray, and air-assisted airless spray.
Air spray uses compressed air to break up the liquid coating formulation into droplets and to propel the droplets to the substrate. The most common type of air nozzle mixes the coating formulation and high-velocity air outside of the nozzle to gauge atomization. Auxiliary air streams are used to modify the shape of the spray. The coating formulation flows through the liquid orifice in the spray nozzle with relatively little pressure drop. Siphon or pressure feed, usually at pressures less than 18 psi, are used, depending upon the viscosity and quantity of coating formulation to be sprayed.
Airless spray uses a high pressure drop across the orifice to propel the coating formulation through the orifice at high velocity. Upon exiting the orifice, the high-velocity liquid breaks up into droplets and disperses into the air to form a liquid spray. Sufficient momentum remains after atomization to carry the droplets to the substrate. The spray tip is contoured to modify the shape of the liquid spray, which is usually a round or elliptical cone or a flat fan. Turbulence promoters are sometimes inserted into the spray nozzle to aid atomization. Spray pressures typically range from 700 to 5000 psi. The pressure required increases with fluid viscosity.
Air-assisted airless spray combines features of air spray and airless spray. It uses both compressed air and high pressure drop across the orifice to atomize the coating formulation and to shape the liquid spray, typically under milder conditions than each type of atomization is generated by itself. Generally the compressed air pressure and the air flow rate are lower than for air spray. Generally the liquid pressure drop is lower than for airless spray, but higher than for air spray. Liquid spray pressures typically range from 200 to 800 psi. The pressure required increases with fluid viscosity.
Air spray, airless spray, and air assisted airless spray can also be used with the liquid coating formulation heated or with the air heated or with both heated. Heating reduces the viscosity of the liquid coating formulation and aids atomization.
Electrostatic forces are commonly utilized with orifice sprays such as air spray, airless spray, and air-assisted airless spray to increase the proportion of liquid coating that is deposited onto the substrate from the liquid spray. This is commonly referred to as increasing the transfer efficiency. This is done by using a high electrical voltage relative to the substrate to impart a negative electrical charge to the liquid. The substrate is electrically grounded. This creates an electrical force of attraction between the liquid spray droplets and the substrate, which causes droplets that would otherwise miss the substrate to be deposited onto it. When the electrical force causes droplets to be deposited on the edges and backside of the substrate, this effect is commonly referred to as wrap around. The substrate should be electrically conducting or be given a conducting surface before being sprayed.
The liquid can be electrically charged at any stage of the spray formation process. It can be charged by applying high electrical voltage and electrical current 1) within the spray gun, by direct contact with electrified walls or internal electrodes before passing through the orifice; 2) as the liquid emerges from the orifice, by electrical discharge from external electrodes located near the orifice and close to the spray; or 3) away from the orifice, by passing the liquid spray through or between electrified grids or arrays of external electrodes before the spray reaches the substrate.
Electrically charging the liquid as it emerges from the orifice is widely used. Usually a short sharp-pointed metal wire, which extends from the spray nozzle to beside the spray, is used as the electrode. When a high electrical voltage is applied to the electrode, electrical current flows from the point of the electrode to the liquid spray, which becomes charged. This method is used for air spray, airless spray, and air-assisted airless spray guns. It is used for both hand spray guns and automatic spray guns. Generally the electrical voltage ranges from 30 to 150 kilovolts. Coating formulations that are sufficiently conductive will leak electrical charge through the fluid to the material supply system; these systems must be isolated from electrical ground so that the system itself becomes electrified. For safety reasons, the voltage of hand spray guns is usually restricted to less than 70 kilovolts and the equipment is designed to automatically shut off the voltage when the current exceeds a safe level. Generally for hand spray guns the useful range of electrical current is between 20 and 100 microamperes and optimum results are obtained with coating formulations that have very low electrical conductivity, that is, very high electrical resistance.
U.S. Pat. Nos. 3,556,411; 3,647,147; 3,754,710; 4,097,000; and 4,346,849 disclose spray nozzles and tips for use in airless spray, including designs and methods of manufacture and methods of promoting turbulence in the atomizing fluid. U.S. Pat. No. 3,659,787 discloses a spray nozzle and use of electrostatics for airless spray. U.S. Pat. Nos. 3,907,202 and 4,055,300 disclose spray nozzles and use of electrostatics for air assisted airless spray. None of these patents uses supercritical fluids as diluents to spray coating formulations.
More information about orifice sprays such as air spray, airless spray, and air-assisted airless spray, about heated orifice sprays, and about electrostatic spraying can be obtained from the general literature of the coating industry and from technical bulletins issued by spray equipment manufacturers, such as the following references:
1. Martens, C. R., Editor. 1974. Technology of Paints, Varnishes and Lacquers. Chapter 36. Application. Robert E. Krieger Publishing Company, Huntington, N.Y.
2. Fair, James. 1983. Sprays. Pages 466-483 in Grayson, M., Editor. Kirk-Othmer Encyclopedia of Chemical Technology. Third Edition. Volume 21. Wiley-Interscience, New York.
3. Zinc, S. C. 1979. Coating Processes. Pages 386-426 in Grayson, M., Editor. Kirk-Othmer Encyclopedia of Chemical Technology. Third Edition. Volume 6. Wiley Interscience, New York.
4. Long, G. E. 1978 (March 13). Spraying Theory and Practice. Chemical Engineering: 73-77.
5. Technical Bulletin. Air Spray Manual. TD10-2R. Binks Manufacturing Company, Franklin Park, Ill.
6. Technical Bulletin. Compressed Air Spray Gun Principles. TD10-1R-4. Binks Manufacturing Company, Franklin Park, Ill.
7. Technical Bulletin. Airless Spray Manual. TD11-2R. Binks Manufacturing Company, Franklin Park, Ill.
8. Technical Bulletin. Airless Spraying. TD11-1R-2. Binks Manufacturing Company, Franklin Park, Ill.
9. Technical Bulletin. Electrostatic Spraying. TD17-1R. Binks Manufacturing Company, Franklin Park, Ill.
10. Technical Bulletin. Hot Spraying. TD42-1R-2. Binks Manufacturing Company, Franklin Park, Ill.
11. Technical bulletin on air-assisted airless spray painting system. Kremlin, Incorporated, Addison, Ill.
Prior to the present invention, electrostatic liquid spray application of coatings, such as lacquers, enamels, and varnishes, by the spray methods discussed above was effected solely through the use of organic solvents as viscosity reduction diluents. However, because of increased environmental concern, efforts have been directed to reducing the pollution resulting from painting and finishing operations. For this reason there is great need for new electrostatic liquid spray technology for application of coatings that diminishes the emission of organic solvent vapors.
U.S. Pat. No. 4,582,731 (Smith) discloses a method and apparatus for the deposition of thin films and the formation of powder coatings through the molecular spray of solutes dissolved in organic and supercritical fluid solvents. The molecular sprays disclosed in the Smith patent are composed of droplets having diameters of about 30 Angstroms. These droplets are more than 10.sup.6 to 10.sup.9 less massive than the droplets formed in conventional application methods that Smith refers to as "liquid spray" applications. Furthermore, the orifice used to produce the molecular sprays is typically in the 1 to 4 micron diameter size range. These orifice sizes are 10.sup.3 to 10.sup.5 times smaller in area than orifices used in conventional "liquid spray" apparatus. This disclosed method of depositing thin films seeks to minimize, and preferably eliminate, the presence of solvent within the film deposited upon a substrate. This result is preferably accomplished through the maintenance of reduced pressure in the spray environment. However, the maintenance of reduced pressures is not feasible for most commercial coating applications. Furthermore, the spray method disclosed by Smith utilizes very high solvent-to-solute ratios, thereby requiring undesirably high solvent usage and requiring prohibitively long application times in order to achieve coatings having sufficient thicknesses to impart the desired durability of the coating. Finally, Smith does not apply electrostatics to his molecular spray process.
U.S. patent application Ser. No. 133.068, filed Dec. 21, 1987, (Hoy et al) discloses a process and apparatus for the liquid spray application of coatings to a substrate wherein the use of environmentally undesirable organic diluents is minimized. The process of the invention comprises:
(1) forming a liquid mixture in a closed system, said liquid mixture comprising: PA0 (2) spraying said liquid mixture onto a substrate to form a liquid coating thereon. The invention is also directed to a liquid spray process as described immediately above to which at least one active organic solvent (c) is admixed with (a) and (b), prior to the liquid spray application of the resulting mixture to a substrate. The preferred supercritical fluid is supercritical carbon dioxide fluid. The apparatus of the invention comprises an apparatus in which the mixture of the components of the liquid spray mixture can be blended and sprayed onto an appropriate substrate. Said apparatus is comprised of, in combination: PA0 (1) forming a liquid mixture in a closed system, said liquid mixture comprising: PA0 (2) spraying said liquid mixture onto a substrate to form a liquid coating thereon by passing the mixture under pressure through an orifice into the environment of the substrate to form a liquid spray; and PA0 (3) electrically charging the liquid by a high electrical voltage relative to the substrate and electric current.
(a) at least one polymeric compound capable of forming a coating on a substrate; and PA1 (b) at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture of (a) and (b) to a point suitable for spray applications; PA1 (1) means for supplying at least one polymeric compound capable of forming a continuous, adherent coating; PA1 (a) at least one polymeric component capable of forming a coating on a substrate; and PA1 (b) a solvent component containing at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application;
(2) means for supplying at least one active organic solvent;
(3) means for supplying supercritical carbon dioxide fluid;
(4) means for forming a liquid mixture of components supplied from (1)-(3);
(5) means for spraying said liquid mixture onto a substrate.
The apparatus further comprises (6) means for heating any of said components and/or said liquid mixture of components. Hoy et al demonstrate the use of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent borne and/or highly viscous non aqueous dispersions coatings compositions to dilute these compositions to application viscosity required for liquid spray techniques. They further demonstrate that the method is generally applicable to all organic solvent borne coatings systems. However, they do not teach the means for spraying and do not apply electrostatics.
Supercritical carbon dioxide fluid is an environmentally safe, non-polluting diluent that allows utilization of the best aspects of organic solvent borne coatings applications and performance while reducing the environmental concerns to an acceptable level. It allows the requirements of shop-applied and field-applied liquid spray coatings as well as factory-applied finishes to be met and still be in compliance with environmental regulations.
Clearly what is needed is an electrostatic liquid spray method of coating substrates that can be applied to using supercritical fluids, such as supercritical carbon dioxide fluid, as diluents to reduce coating formulations to spray viscosity. Such a method should utilize the properties of the supercritical fluid, should be compatible with existing spray technology and practice, and should be environmentally acceptable.
Prior to the present invention, it was unknown if electrostatics could be used with polymeric liquid spray mixtures that contain a high concentration of highly volatile supercritical fluid like supercritical carbon dioxide fluid. It was surmised that the spray mixture would be too electrically conductive to apply a high electrical voltage to without having to electrically isolate the material supply and fluid delivery equipment, which are normally electrically grounded, to prevent leakage of electrical charge from the spray. Measurement of the electrical conductivity of supercritical or liquid carbon dioxide could not be found in the literature to predict the effect on the conductivity of the spray mixture. It was expected that the rapid volatization of the supercritical carbon dioxide fluid from the spray (upon exiting the orifice) would create a strong enough counter flow to blow the charging electrical current, coming from the external electrode, away from the spray and prevent the spray from becoming electrically charged. Alternatively, it was anticipated that the counter flow would reduce or limit the electrical charge level that could be applied to the spray. If the liquid spray droplets were charged, it was considered likely that volatization of the supercritical fluid dissolved in the droplets would increase the rate of loss of the electrical charge from the droplets and thereby reduce the electrical attraction between the droplets and the substrate, which would reduce transfer efficiency and electrical wrap around. Furthermore, rapid cooling of the spray caused by depressurization of the supercritical fluid predictably would lower spray temperature to below the dew point and condense moisture onto the droplets, which would also increase the rate of electrical charge loss from the droplets. It was expected that the expansion of the supercritical fluid from the spray would enhance the amount of coating material that issues from the periphery of the spray as electrically charged mist, which would be electrically deposited onto surrounding objects, such as the operator, instead of on the substrate. This result might be hazardous to the operator and prevent the safe use of electrostatic hand spraying. Finally, it was expected that the supercritical fluid spray, which tends to widen more than normal sprays, would hit the external electrode and deposit spray material onto it. The deposited material would be entrained into the spray as large drops or foam and damage the coating on the substrate. The deposited material might also interfere with the charging current given off from the electrode and thereby prevent charging of the spray. If the electrode were moved farther away, to keep the spray from hitting it, it would probably be too far away to effectively charge the spray.
Surprisingly, however, it has been discovered that electrostatic liquid sprays can be formed by using supercritical fluids as viscosity reduction diluents, that the electrical forces can be used to increase the proportion of coating formulation that is deposited onto the substrate, and that such electrostatic sprays can be used to deposit quality coherent polymeric coatings onto substrates.
It is accordingly an object of the present invention to demonstrate the use of electrostatic orifice sprays, such as airless spray and air-assisted airless spray, to apply liquid coatings to substrates by liquid sprays in which supercritical fluids, such as supercritical carbon dioxide fluid, are used as diluents in highly viscous organic solvent borne and/or highly viscous non-aqueous dispersions coatings compositions to dilute these compositions to application viscosity.
It is also an object of the present invention to demonstrate the use of electrostatic forces with said orifice sprays, coating compositions, and supercritical fluid diluents to increase the proportion of liquid coating that is deposited onto the substrate from the spray.
A further object of the invention is to demonstrate that the method is generally applicable to all organic solvent borne coatings systems.
These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein set forth.